This section provides background information and introduces information related to various aspects of the disclosure that are described and/or claimed below. These background statements are not admissions of prior art.
Techniques involving substrate patterning exploit the fact that the threading dislocations are constrained by geometry, i.e. that a dislocation cannot end in a crystal. If the free edge is brought closer to another free edge by patterning the substrate into smaller growth areas, then it is possible to reduce threading dislocation densities. In the past, a combination of substrate patterning and epitaxial lateral overgrowth (“ELO”) techniques was demonstrated to greatly reduce defect densities in gallium nitride device, leading to fabrication of laser diodes with extended lifetimes. This process substantially eliminates defects in ELO regions but highly defective seed windows remain, necessitating repetition of the lithography and epitaxial steps to eliminate all defects. In a similar approach, pendeo-epitaxy eliminates substantially all defects in the epitaxial region proximate to the substrate but requires one lithography and two epitaxial growth steps. Furthermore, both techniques require the increased lateral growth rate of gallium nitride, which has not been demonstrated in all heteroepitaxial systems.
Another known technique termed “epitaxial necking” was demonstrated in connection with fabricating a Ge-on-Si heterostructure by Langdo et al. in “High Quality Ge on Si by Epitaxial Necking,” Applied Physics Letters, Vol. 76, No. 25, April 2000. This approach offers process simplicity by utilizing a combination of selective epitaxial growth and defect crystallography to force defects to the sidewall of the opening in the patterning mask, without relying on increased lateral growth rates. Specifically, in the (111)<110> diamond cubic slip system, misfit dislocations lie along <110> directions in the (100) growth plane while the threading segments rise up on (111) planes in <110> directions. Threading segments in <110> directions on the (111) plane propagate at a 45° angle to the underlying Si (100) substrate surface. Thus, if the aspect ratio of the holes in the patterning mask is greater than 1, threading segments will be blocked by the mask sidewall, resulting in low-defect top Ge “nodules” formed directly on Si. One important limitation of epitaxial necking, however, is the size of the area to which it applies. In general, as discussed in more detail below, the lateral dimensions in both dimensions have to be relatively small in order for the dislocations to terminate at sidewalls.
Thus, there is a need in the art for versatile and efficient methods of fabricating semiconductor heterostructures that would constrain dislocation defects in a variety of lattice-mismatched materials systems. There is also a need in the art for semiconductor devices utilizing a combination of integrated lattice-mismatched materials with reduced levels of dislocation defects for improved functionality and performance.